I've set /etc/passwd so that /bin/bash is my shell for both the normal user as well as root. I've set up /etc/profile with a number of parameters i need (ex PATH, umask, PS1). When i log in with my user, i am assiging those settings. If i 'su -', i am also assigned those env settings.
However, if i just 'su' i lose them and get a default PATH of /usr/sbin:/usr/bin. interestingly, umask setting stays put.